<!DOCTYPE html>
<html lang="zh-cn">
    <head>
        <meta charset="UTF-8"/>
        <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0" />
        <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />
        <title>Document</title>
        <style>
            *{
                padding: 0;
                margin: 0;
            }
            html,body{
                height: 100%;
            }
            .box{
                height: 100%;
                width: 100%;
            }
            .df{
                display: -webkit-flex;
                display: -moz-flex;
                display: -ms-flex;
                display: -o-flex;
                display: flex;
            }
            .f1{
                flex:1;
            }
            .bg-red{
                background-color: #f00;
            }
            .bg-green{
                background-color: #0f0;
            }
            .bg-blue{
                background-color: #00f;
            }
            .h-100p{
                height: 100%;
            }
            .box-100p-80{
                width: 100%;
                height: 80px;
            }
            .poa{
                position: absolute;
            }
            .b0{
                bottom: 0;
            }
            .fd-c{
                -webkit-flex-direction: column;
                -moz-flex-direction: column;
                -ms-flex-direction: column;
                -o-flex-direction: column;
                flex-direction: column;
            }
        </style>
    </head>
    <body>
        <div class="box bg-green df fd-c">
            <div class="df f1 bg-red">123</div>
            <div class="box-100p-80 poa b0 bg-blue"></div>
        </div>
    </body>
</html>